Silicon bipolar transistor, circuit arrangement and method for production of a silicon bipolar transistor

ABSTRACT

The silicon bipolar transistor ( 100 ) comprises a base, with a first highly-doped base layer ( 105 ) and a second poorly-doped base layer ( 106 ) which together form the base. The emitter is completely highly-doped and mounted directly on the second base layer ( 106 ).

[0001] The invention relates to a silicon bipolar transistor, a circuitarrangement and also a method for producing a silicon bipolartransistor.

[0002] A silicon bipolar transistor, such a circuit arrangement and sucha method of production are known from [1].

[0003] A normal silicon bipolar transistor has an emitter, a base and acollector.

[0004] In the bipolar transistor known from [1], the maximum oscillatingfrequency of a bipolar transistor is known to be given in accordancewith the following specification: $\begin{matrix}{{f_{\max} \propto \sqrt{\frac{f_{T}}{6 \cdot \Pi \cdot R_{B} \cdot C_{BC}}}}\quad,} & (1)\end{matrix}$

[0005] where

[0006] f_(max) denotes the maximum oscillating frequency of the bipolartransistor,

[0007] f_(T) denotes the transit frequency of the bipolar transistor,

[0008] R_(B) denotes the base resistance of the bipolar transistor,

[0009] C_(BC) denotes the base/collector capacitance of the bipolartransistor.

[0010] As can be seen from [1], it is thus desirable to reduce the baseresistance R_(B) of a bipolar transistor in order to obtain the highestpossible oscillating frequency for the bipolar transistor.

[0011] The base resistance R_(B) of a bipolar transistor is determinedboth by the electrical resistance of the connection region and by thesheet resistance of the base doping profile with doping atoms.

[0012] When the transistor base is homogeneously doped with dopingatoms, the sheet resistance, which is also called the pinch, isinversely proportional to the layer thickness of the base.

[0013] However, increasing the layer thickness of the base of thebipolar transistor results in an increase in the base transit time τ forthe minority carriers in the bipolar transistor.

[0014] Increasing the doping of the base with doping atoms beyond aconcentration of 5×10¹⁸ cm⁻³ causes a reduction in the breakdown voltageof the junction between the emitter and the base of the bipolartransistor to excessively low values and at the same time increases thebase/emitter depletion layer capacitance.

[0015] To reduce the base resistance, [1] makes provision for theemitter of the bipolar transistor to be lightly doped with doping atomsin a concentration of approximately 10¹⁸ cm⁻³.

[0016] By contrast, the base of the bipolar transistor known from [1] isheavily doped with doping atoms in a doping atom concentration ofapproximately 10²⁰ cm⁻³.

[0017] The effect achieved by this is that, when the emitter is lightlydoped, the base can be heavily doped without losing the blocking abilityof the emitter/base junction of the bipolar transistor.

[0018] To increase the transit frequency, the base of the transistordescribed in [1] contains germanium.

[0019] In addition, [2] describes reduction of the boron diffusion byadding carbon atoms for a transistor having an epitaxial emitter.

[0020] In addition, [3] discloses a bipolar transistor which has a veryhigh maximum oscillating frequency f_(max) of 74 GHz.

[0021] [6] describes a bipolar transistor based on gallium arsenide,where a base layer comprising two partial base layers, a p⁺⁺-doped firstbase layer and a p⁺-doped second base layer respectively made of galliumarsenide is applied on a collector layer made of n-doped galliumarsenide.

[0022] The p⁺-doped gallium arsenide partial base layer serves asdiffusion barrier for the zinc doping atoms. The p⁺-doped galliumarsenide layer has an n⁺-doped emitter stop layer applied on it whichserves as barrier layer between the emitter and the base for insulatingthe zinc, the intention being to continue to ensure that the emitter isn-doped. The emitter stop layer has an n-doped “graded” emitter layersequence made of gallium arsenide applied on it.

[0023] In addition, [7] illustrates a silicon bipolar transistorcontaining a base which has two base layers, a first, p⁺-doped baselayer being applied on an n-doped collector, and a p⁻-doped second baselayer being applied on this base layer. The second base layer has ann-doped, i.e. lightly doped, first intermediate layer applied on it, andonly then is the heavily doped n⁺-emitter applied on said firstintermediate layer.

[0024] This layer sequence has the particular drawback that an n-dopedintermediate layer needs to be inserted between the base and theemitter, which means considerable complexity for production,particularly in mass production, and hence creates considerabletechnological difficulties. In addition, the production costs in massproduction are very high for such a transistor.

[0025] [8] describes a power transistor in which the base layer containsa further layer of the same conductivity type as the base layer.

[0026] [9] describes a transistor in which the emitter zone has a zoneof the base zone's conduction type arranged before it, the defectconcentration of said zone being lower than that of the base zone, butamounting to at least 10¹⁶ defects per cm³.

[0027] [10] describes a semiconductor apparatus in which an emitterlayer, an intrinsic base layer surrounding the emitter layer, with thesurface of the emitter layer permitting exposure, external base layersand link base layers situated between the intrinsic base layer and theexternal base layers are formed on a collector layer.

[0028] [11] describes a transistor for switching with a partiallyfalling characteristic and a semiconductor body having the zone sequencenpp⁺n⁺ or pnn⁺p⁺.

[0029] [12] describes a bipolar transistor and a method for productionthereof, the bipolar transistor having a collector region and insulatingregions which surround the latter. Arranged above the collector regionis a monocrystalline layer sequence, and arranged above the insulatingregions is a polycrystalline layer sequence, a cover layer beingarranged above the base layer, and part or all of the cover layer beingremoved in the active emitter region.

[0030] Other GaAs bipolar transistors are described in [13], [14] and[15].

[0031] The invention is thus based on the problem of specifying asilicon bipolar transistor, a circuit arrangement and also a method forproducing a silicon bipolar transistor where the silicon bipolartransistor has a higher maximum oscillating frequency f_(max) ascompared with a silicon bipolar transistor in accordance with [3].

[0032] The problem is solved by the silicon bipolar transistor, thecircuit arrangement and also by the method for producing a siliconbipolar transistor having the features in accordance with theindependent patent claims.

[0033] A silicon bipolar transistor has an emitter, a base and acollector. The entire emitter is heavily doped with doping atoms, thedoping atoms being of opposite conductivity type to the doping atomsused for doping the base regions. This means that, if the base regionsare n-doped, the entire emitter is heavily p-doped, and if the baseregions are p-doped, the entire emitter is heavily n-doped. The emitterpreferably contains polysilicon.

[0034] The base is grouped into a first base region and a second baseregion, the second base region being doped with doping atoms, forexample with boron atoms, in a low concentration, i.e. the second baseregion is lightly doped with doping atoms.

[0035] By contrast, the first base region is heavily doped with thedoping atoms, for example with boron atoms.

[0036] The terms “light doping” and “heavy doping” should be understood,within the context of this invention, as meaning that the number ofdoping atoms per cm³ is much greater in the case of heavy doping ascompared with light doping, preferably greater by at least a factor oftwo.

[0037] By way of example, the second base region may have a doping of5×10¹⁷ to 1×10¹⁹ doping atoms per cm³, and the first base region mayhave a doping of 10¹⁹ to approximately 2×10²⁰ doping atoms per cm³.

[0038] The invention can clearly be seen in that a bipolar transistor'sbase, which is normally doped as homogeneously as possible, is splitinto a first region, the first base region, with heavy doping and asecond region, the second base region, with light doping.

[0039] In this way, the sheet resistance of the base, i.e. the baseresistance R_(B), is significantly reduced, and may even be reduced bymore than a factor of 5.

[0040] In accordance with one refinement of the invention, the widths ofthe first base region (first base width W1) and of the second baseregion (second base width W2) can be proportioned in accordance with thefollowing specifications:

[0041] The second base width W2 preferably has a width of 10 nm to 40nm, which is chosen on the basis of the desired reverse voltage of theemitter/base pn junction, for example a second base width W2 of 20 nmfor a reverse voltage of 2 V.

[0042] The first base width W1 is chosen to be as thin as possible. Inaddition, the first base region is doped as heavily as possible, so thatno severe widening of the profile occurs during subsequent temperaturesteps. The first base width W1 may be 1 nm to 30 nm, for example.

[0043] The first base region is situated at the collector of the bipolartransistor and, as can be seen from the specifications illustratedabove, is preferably formed so as to be as narrow as possible, i.e. withas small a first base width W1 as possible, and is doped as heavily aspossible with doping atoms.

[0044] To reduce the diffusion of the individual doping atoms betweenthe first base region and the second base region further, it isadvantageous, in accordance with one refinement of the invention, tosupply carbon atoms to the base in order to reduce the diffusion of the,by way of example, boron doping atoms.

[0045] In accordance with one refinement of the invention, a transistorcurrent gain possibly decreasing with rising base charge canadvantageously be compensated for by adding germanium atoms. Inaddition, adding Ge further increases the transit frequency of thebipolar transistor, and hence also the maximum oscillation frequency.

[0046] In accordance with another refinement of the invention, provisionis made for the second base region to be doped with a concentration ofapproximately 5×10¹⁸ cm⁻³, and for the first base region to be dopedwith 3×10¹⁹ cm⁻³ doping atoms.

[0047] As described in [3], the emitter window (i.e. the region in whichthe emitter is to be formed) is opened using dry etching in a sandwichstructure. Viewed from bottom to top, the sandwich structure contains:

[0048] p⁺-polysilicon,

[0049] TEOS,

[0050] nitride.

[0051] The side wall of the emitter window is produced by a nitridespacer.

[0052] The collector, which is initially still covered by an oxidelayer, is exposed by means of isotropic wet etching. In this case, apolysilicon overhang is produced by under-etching the polysiliconsituated above, as described in [4].

[0053] In addition, in accordance with one preferred refinement of theinvention, provision is made for aluminium atoms or else gallium atomsto be used as the doping atoms instead of boron atoms.

[0054] However, the use of boron atoms has the advantage that the boronatoms normally have a lower diffusion speed than the further knowndoping atoms, which may of course be used as an alternative, and this isparticularly advantageous for producing the two base regions withgreatly different doping.

[0055] A circuit arrangement having at least one such bipolar transistoris particularly suitable for use in radio-frequency applications, forexample in the mobile radio sector or generally in processors with ahigh clock rate.

[0056] In a method for producing a bipolar transistor, after insulation,i.e. after the collector has been formed, a first base layer, whichforms the first base region, is grown on the collector, preferably bymeans of vapour phase epitaxy using a first partial pressure, forexample using diborane (B₂H₆) as the doping gas.

[0057] It should be pointed out that the incorporation of the dopingatoms is, to a first approximation, linear to the partial pressure usedduring the vapour phase epitaxy.

[0058] On the first base layer, a second base layer, which forms thesecond base region, is grown by means of vapour phase epitaxy using asecond partial pressure, the second partial pressure being much lowerthan the first partial pressure, and the use of diborane as the dopinggas, likewise within the context of the vapour phase epitaxy for thesecond base layer, meaning that the second base layer and hence thesecond base region have a much lighter doping than the first base layer,i.e. the first base region.

[0059] In accordance with one refinement of the invention, as the baseis formed, germanium is added in accordance with the procedure describedin [5], the invention ensuring that the step profile, formed by thefirst base region and the second base region, is formed appropriately,as subsequently described in connection with FIG. 3. The emitter isapplied directly on the second base region. The entire emitter isheavily doped with doping atoms.

[0060] Exemplary embodiments of the invention are illustrated in thefigures and are explained in more detail below.

[0061]FIG. 1 shows a cross-section through a bipolar transistor inaccordance with an exemplary embodiment of the invention;

[0062]FIGS. 2a to 2 c show cross-sections through the structure of thebipolar transistor at different production instants; and

[0063]FIG. 3 shows a sketch of the doping profile of the bipolartransistor from FIG. 1.

[0064]FIG. 1 shows a bipolar transistor 100 having a base connection101, an emitter connection 102 and a collector connection 103.

[0065] The base connection 101 is coupled to two base regions formingthe base by means of a p-doped polysilicon layer 104.

[0066] A first base region 105 has a heavy doping of 3×10¹⁹ dopingatoms, of boron atoms in accordance with this exemplary embodiment.

[0067] A second base layer 106, as the second base region, is grown onthe first base region 105 by means of vapour phase epitaxy, the dopingof the second base region 106 being approximately 5×10¹⁸ doping atomsper cm³.

[0068] The base, in particular the first base region 105, is grown on acollector layer 107 by means of vapour phase epitaxy, as explained inmore detail below. An n⁺-doped layer 108 is embedded in the collectorlayer (n⁺-buried layer).

[0069] The n⁺-doped layer 108 couples the collector 107, i.e. thecollector layer 107, to the collector connection 103.

[0070] The method described below for producing the bipolar transistor,as shown in FIG. 2a to FIG. 2c, essentially corresponds to theproduction method as described in [3] for a bipolar transistor with ahomogeneously doped base.

[0071] However, a difference in the production method is provided withinthe context of formation of the base layer.

[0072] Starting from a collector layer 107 containing silicon, theemitter region, i.e. the region in which the emitter is to be formed atthe end of the production method, is defined, as described in [3], bymeans of a sandwich structure formed on an oxide layer which is formedfrom the vapour phase using a deposition method (CVD method).

[0073] Viewed from bottom to the top, the sandwich structure containsthe following layers:

[0074] p⁺-polysilicon,

[0075] TEOS,

[0076] nitride.

[0077] The side wall of the emitter window is produced by a nitridespacer.

[0078] The collector, which is initially still covered by an oxidelayer, is exposed by means of isotropic wet etching. In this case, apolysilicon overhang is produced by under-etching the polysiliconsituated above, as described in [4].

[0079] The collector layer 107 is n-doped with 2×10¹⁷ cm⁻³ doping atoms.

[0080] After thin nitride spacers 203 have been formed on the sandwichstructure 201, the oxide layer 202 is under-etched below the p⁺-dopedpolysilicon layer 204 using wet etching, so that contact regions 205having a width of approximately 0.1 μm are produced.

[0081] In a further step (cf. FIG. 2b), vapour phase epitaxy is used ata temperature of 650° C. to 900° C. and a pressure of 1 to 100 torr togrow the base layer 206, formed by the first base layer 207 and thesecond base layer 208.

[0082] In accordance with this exemplary embodiment, the gases usedwithin the context of the vapour phase epitaxy are a hydrogen carriergas at 10 to 50 slm, containing the following gases for incorporatingcarbon atoms and germanium atoms in order to obtain the properties ofthe bipolar transistor and to reduce the diffusion of the doping atoms(described below) between the first base region and the second baseregion:

[0083] dichlorosilane (SiH₂Cl₂)

[0084] hydrochloric acid (HCl),

[0085] germane (GeH₄),

[0086] methylsilane (SiH₃CH₃),

[0087] which gases are used within the context of the vapour phaseepitaxy at a partial pressure of 10⁻⁴-10⁻² of the total pressure usedwithin the context of the vapour phase epitaxy.

[0088] The doping gas used for forming the first base layer 207 isdiborane (B₂H₆) at a partial pressure of 10⁻⁵ of the total pressure. Inthis context, germane is added to the first base layer at a partialpressure of 10⁻⁴, so that the first base layer has a concentration ofgermanium atoms of approximately 20% with 3×10¹⁹ boron atoms, so thatthe doping profile described in [5], for example, is produced forgermanium, as shown in FIG. 3.

[0089] On account of the dependency of the concentration of the dopingbeing, to a first approximation, linear to the partial pressure, adoping of doping atoms in the first base layer 207 is produced which isgreater by at least a factor of 2 than the concentration, i.e. thedoping with doping atoms, in the second base layer 208.

[0090] The first base width W1 is chosen to be as thin as possible. Inaddition, the first base region is doped as heavily as possible, so thatno severe widening of the profile occurs during subsequent temperaturesteps. In accordance with this exemplary embodiment, the first basewidth W1 is 1 nm to 30 nm.

[0091] On the first base layer 207, diborane (B₂H₆) is used to form thesecond base layer 208 at a partial pressure of 10⁻⁶ of the totalpressure.

[0092] In turn, germane is added in accordance with the profile shown inFIG. 3 during formation of the second base layer 208, at a partialpressure of 10⁻⁵ of the total pressure.

[0093] This produces the second base layer 208 with a dopingconcentration of approximately 5×10¹⁸ boron doping atoms per cm³ andapproximately 5% germanium atoms.

[0094] In accordance with this exemplary embodiment, the second basewidth W2 has a width of 10 nm to 40 nm, which is chosen on the basis ofthe desired reverse voltage of the emitter/base pn junction, and has asecond base width W2 of 20 nm for a reverse voltage of 2 V.

[0095] In a further step (cf. FIG. 2c), the nitride spacers 203 areremoved using phosphoric acid, and, in accordance with the methoddescribed in detail in [3], the n⁺-doped polysilicon emitter 209 isformed on further spacers 210, which are in turn grown on the secondbase layer 208.

[0096] In accordance with this procedure, the doping profile 300 shownin FIG. 3 is produced to form the bipolar transistor 100 from FIG. 1.

[0097] Along the ordinate 301, which describes the local orientationagainst the direction of growth of the individual layers within thebipolar transistor 100, the respective concentration of the doping atomsin the respective layer is shown using the abscissa 302.

[0098] It is advantageous to design the first base region to be asnarrow as possible and to dope it heavily in order to keep down thetransit time of the electrons via the base.

[0099] On the basis of an emitter doping curve 303, which shows thedoping concentration of the doping atoms in the emitter layer 209, asecond base doping profile 304 along the second base width W2 is thenused to show the profile of the doping of boron atoms in the second baselayer 208 of 5×10¹⁸ boron atoms per cm³, which second base layer mergesin stepped fashion, i.e. essentially abruptly, into a heavy doping inthe first base region, i.e. the first base layer 207 with the first basewidth W1, in which a doping of boron atoms of 3×10¹⁹ boron atoms per cm³(symbolized by the first base doping profile 305) is provided.

[0100] Dashed lines 306 show the corresponding concentration profile forgermanium atoms in the first base layer 207 and second base layer 208 inthe base. The plateau region, i.e. the second base region, containsapproximately 5% germanium atoms. The collector-side region, i.e. thefirst base region, contains approximately 20% germanium atoms.

[0101] Experiments have shown that such a bipolar transistor having thedoping profile shown above halves a sheet resistance for the base of,normally, 7 kΩ to 3.5 kΩ by using the base profile, with the transittime τ of the bipolar transistor of 1.5 ps for a homogeneous baseincreasing only insignificantly to a transit time of 1.6 ps for a splitbase with different dopings.

[0102] This document cites the following publications:

[0103] [1] B. Heinemann et al., Influence of low doped emitter andcollector regions on high-frequency performance of SiGe-Base HBTs, SolidState Electronics, Vol. 38, No. 6, pp. 1183-1189, 1995

[0104] [2] D. Knoll et al, Si/SiGe:C Heterojunction Bipolar Transistorsin an Epi-Free Well, Single-Polysilicon Technology, IEDM 98, pp.703-706, 1998

[0105] [3] T. F. Meister et al., SiGe Base Bipolar Technology with 74GHz f_(max) and 11 ps Gate Delay, IEDM, pp. 739-740, 1995

[0106] [4] U.S. Pat. No. 5,326,718

[0107] [5] Niu Guofu et al, Noise Parameter Modeling and SiGe ProfileDesign Tradeoffs for RF Applications, Proc. Of the 2nd Topical Meetingon Silicon Monolithic Integrated Circuits in RF Systems, pp. 9-14, 2000

[0108] [6] U.S. Pat. No. 5,132,764

[0109] [7] U.S. Pat. No. 5,177,583

[0110] [8] DD 230 677 A3

[0111] [9] DE-OS 151 48 48

[0112] [10] DE 42 40 205 A1

[0113] [11] DE-AS 1 089 073

[0114] [12] DE 198 45 789 A1

[0115] [13] U.S. Pat. No. 4,593,305

[0116] [14] JP 03-280 546

[0117] [15] JP 03-192 727

1. Silicon bipolar transistor, having a base, an emitter, the entiretyof which is heavily doped with doping atoms, a collector, where the basehas a first base region and a second base region, where the first baseregion is heavily doped with doping atoms, where the second base regionis lightly doped with doping atoms, and where the emitter is applieddirectly on the second base region.
 2. Silicon bipolar transistoraccording to claim 1, in which the first base region is arranged closerto the collector than the second base region.
 3. Silicon bipolartransistor according to claim 1 or 2, in which the first base region ismore heavily doped with doping atoms than the second base region by atleast a factor of
 2. 4. Silicon bipolar transistor according to claim 3,in which the second base region has a doping of approximately 5×10¹⁸doping atoms per cm³.
 5. Silicon bipolar transistor according to claim 3or 4, in which the first base region has a doping of approximately3×10¹⁹ doping atoms per cm³.
 6. Silicon bipolar transistor according toone of claims 1 to 5, in which the second base region has a base widthof between 10 nm and 40 nm.
 7. Silicon bipolar transistor according toone of claims 1 to 6, in which the first base region has a base width ofbetween 1 nm and 30 nm.
 8. Silicon bipolar transistor according to oneof claims 1 to 7, in which the base additionally has further dopingatoms which are used to support an essentially abrupt junction betweenthe first base region and the second base region.
 9. Silicon bipolartransistor according to one of claims 1 to 8, in which the doping atomscontain boron atoms.
 10. Silicon bipolar transistor according to one ofclaims 1 to 9, in which the base contains further carbon atoms forreducing the diffusion of the doping atoms.
 11. Silicon bipolartransistor according to one of claims 1 to 10, in which the basecontains germanium atoms.
 12. Silicon bipolar transistor according toone of claims 1 to 11, in which the emitter contains polysilicon. 13.Circuit arrangement having at least one silicon bipolar transistoraccording to one of claims 1 to
 12. 14. Method for producing a siliconbipolar transistor, in which a collector is formed, in which a base isformed, in which a first base region is heavily doped with doping atoms,in which a second base region is lightly doped with doping atoms, inwhich an emitter is formed directly on the second base region, and inwhich the entire emitter is heavily doped with doping atoms.